The size limits that are available for individual messages can be divided into the following basic categories:

• Message header size limits   These limits apply to the total size of all message header fields that are present in a message. The size of the message body or attachments is not considered. Because the header fields are plain text, the size of the header is determined by the number of characters in each header field, and by the total number of header fields. Each character of text consumes 1 byte.
  

  Note:
  Some third-party firewalls or proxy servers apply their own message header size limits. These third-party firewalls or proxy servers may have difficulty processing messages that contain attachment file names that are greater than 50 characters, or attachment file names that contain non-US-ASCII characters.

• Message size limits   These limits apply to the total size of a message. This includes the message header, the message body, and any attachments. Message size limits may be imposed on incoming messages or outgoing messages. For internal message flow, Exchange 2007 uses the custom X-MS-Exchange-Organization-OriginalSize: message header to record the original message size of the message as it enters the Exchange 2007 organization. Whenever the message is checked against the specified message size limits, the lower value of the current message size or the original message size header is used. The size of the message can change because of content conversion, encoding, and agent processing.
  
• Attachment size limits   These limits apply to the maximum allowed size of a single attachment within a message. The message may contain many attachments that greatly increase the overall size of the message. However, an attachment size limit would apply to the size of an individual attachment only.
  
• Recipient limits   These limits apply to the total number of message recipients. When a message is first composed, the recipients exist in the To:, Cc:, and Bcc: header fields. When the message is submitted for delivery, the message recipients are converted into RCPT TO: entries in the message envelope. A distribution group is counted as a single recipient during message submission.
  

The scope of the limits that are available for individual messages can be divided into the following basic categories:

• Organizational limits   These limits apply to all Exchange 2007 servers that exist in the organization. The specified message limits apply to all Exchange 2007 servers that have the Hub Transport server role installed. On an Edge Transport server, the specified limits apply to the specific server.
  
• Global limits   These limits apply to all Exchange 2007 and Exchange Server 2003 servers that exist in the organization. The global message limits are stored in the Active Directory directory service.
  In the release to manufacturing (RTM) version of Microsoft Exchange Server 2007, it is common for the organization limits and the global limits to conflict. When the organizational limits and the global message limits conflict, the lowest value takes precedence. In Exchange 2007 RTM, you must use Exchange System Manager on an Exchange 2003 server or the Active Directory Service Interfaces (ADSI) Edit tool to modify global message limits. For more information, see How to Modify Exchange 2003 Global Message Size Limits in Exchange 2007 RTM.
  In Microsoft Exchange Server 2007 Service Pack 1 (SP1), the condition that cause the organization limits and the global limits to conflict has been eliminated. Changes that you make to the organizational limits are automatically copied to the corresponding global limits. In Exchange 2007 SP1, you can modify the organizational limits by using the Set-TransportConfig cmdlet in the Exchange Management Shell, or by configuring the Hub Transport server organization configuration properties in the Exchange Management Console.
  
• Connector limits   These limits apply to any messages that use the specified Send connector, Receive connector, or Foreign connector for message delivery. Connectors are defined on Hub Transport servers or Edge Transport servers.
  In Exchange 2007 SP1, you can also set message size limits on the following types of connections:
  
  • Active Directory site links   Hub Transport servers use Active Directory sites and the costs that are assigned to the Active Directory IP site links to determine the least cost routing path from each Hub Transport server in the organization to every other Hub Transport server in the organization. You can use the Set-AdSiteLink cmdlet to assign an Exchange-specific cost to the Active Directory IP site link. The Exchange-specific cost is a separate attribute that is used instead of the Active Directory-assigned cost to determine the least cost routing path. Any message size limits that are specified on an Active Directory site link don't affect the selection of the least cost routing path. Any messages that exceed the maximum message size limit on any Active Directory site link included in the least cost routing path won't be delivered, and will generate a delivery status notification (DSN) that has the value 5.3.4. For more information, about message routing in Exchange 2007, see Planning to Use Active Directory Sites for Routing Mail.
    
  • Routing group connectors   A routing group connector is used to send and receive messages between Exchange 2007 Hub Transport servers and Exchange 2003 or Exchange 2000 bridgehead servers when the organization is running more than one version of Exchange Server. Any message size limits that are specified on a routing group connector don't affect the selection of the least cost routing path. Any messages that exceed the maximum message size limit on any routing group connector in the least cost routing path won't be delivered. They will generate a DSN that has the value 5.3.4. For more information about routing group connectors, see Message Routing in a Coexistence Environment.
    
  For more information about message size limits on Active Directory site links and routing group connectors in Exchange 2007 SP1, see How to Configure Message Size Limits for Internal Routing.
  
• Server limits   These limits apply to a specific Hub Transport server or Edge Transport server. The specified message limits are not stored in the Active Directory directory service. You can set the specified message limits independently on each Hub Transport server or Edge Transport server.
  Message size limits can apply to Microsoft Office Outlook Web Access on a Client Access server. For more information, see How to Manage Maximum Message Size in Outlook Web Access.
  
• User limits   These limits apply to a specific user object, such as a mailbox, contact, distribution group, or public folder.
  

Organizational Limits

Global Limits

Connector Limits

Server Limits

User Limits

You can set different message size limits for the same message component, such as the maximum message size, at different levels in the Exchange organization. When different message size limits are applied to the same message component, Exchange 2007 uses an order of precedence to enforce the message size limit that is imposed on the same message component.

For example, the recipient will receive a message that was sent by a particular sender if the following conditions are true:

• An organization-wide send and receive message size limit of 10 MB is imposed on the Exchange 2007 organization.
  
• A particular sender's mailbox has a send message size limit of 20 MB.
  
• The recipient's mailbox has a receive message size limit of 20 MB.
  

Generally, it is better to maximize the restrictiveness of your message size limits. You should base any exceptions on a proven need to exceed the established size limits, and you should put those limits as close as possible to the objects that must exceed the established size limits. This strategy helps make sure that messages in the transport pipeline are rejected as early as possible if they violate message size limits. It is a waste of system resources to set a high message size limit at the Exchange organization level, allow a message to enter the Exchange organization, and then reject the message at the last stage of delivery because of a violation of a message size limit.

The tables in the following sections present scenarios that demonstrate how you can apply various message size limits in Exchange 2007 RTM and in Exchange 2007 SP1.

Internet to Edge Transport Server

Edge Transport Server to Hub Transport Server

Hub Transport Server to Hub Transport Server in the Same Active Directory Forest

Hub Transport Server to Hub Transport Server in Different Active Directory Forests

Mailbox Server to Hub Transport Server

Submission by a Foreign Connector

Submission by the Pickup Directory

Submission by the Replay Directory

Hub Transport Server to Edge Transport Server

Edge Transport Server to the Internet

Messages That Are Generated by Transport Servers

The primary difference in message size limits between Microsoft Exchange Server 2003 and Exchange Server 2007 is in the handling of recipient limits. Exchange 2007 treats a distribution group as one recipient. Exchange 2003 treats each member of the expanded distribution list as one recipient. This change was implemented to avoid the partial message delivery scenarios that may occur in Exchange 2003.

Partial message delivery occurs in Exchange 2003 if the number of individual recipients and the recipients that are contained within the distribution list exceeds the specified recipient limit. The total number of message recipients isn't known until after distribution list expansion. Message delivery occurs as the distribution list is expanded until the number of recipients reaches the specified limit. The remaining recipients don't receive the message, but at least the sender receives a non-delivery report (NDR) for each unsuccessful delivery. However, if delivery failure reporting is disabled for the distribution list, the remaining recipients wouldn't receive the message, and the sender would not know who didn't receive the message.

For more information, see the following topics:

• Send Connectors
  
• Receive Connectors
  
• Overview of Journaling
  
• Managing Delivery Status Notifications
  
• Set-TransportConfig
  
• Set-TransportServer